Engines in which natural gas is directly injected into combustion chambers have achieved limited commercial success in the trucking industry. Diesel-cycle engines are employed in this industry due to their high thermal efficiency which is the highest of any internal combustion engine. The thermal efficiency results from high compression ratios that can be in the range of 15:1 to 22:1, as compared to a typical compression ratio for a gasoline engine of 10:1. It has been discovered that when natural gas is injected late in the compression stroke and ignited the power produced is comparable to when these engines burn diesel fuel, but with lower overall emissions. Considering also the lower cost of natural gas compared to diesel, the adoption of natural gas in the trucking industry has seen steady but limited growth.
Natural gas, whose primary constituent is methane, has a much higher auto-ignition temperature than diesel fuel. As used herein, the auto-ignition temperature is the lowest temperature at which a substance will spontaneously ignite without an external ignition source, such as a flame or a spark. For example, the auto-ignition temperature of normal diesel fuel in a normal atmosphere is approximately 210° C., and for natural gas it is approximately 540° C. depending upon the fuel quality. Due to the difference in auto-ignition temperatures, natural gas does not reliably ignite from the heat of compression like diesel fuel for the range of compression ratios previously mentioned. As a result an ignition source is required to ignite natural gas in a Diesel-cycle engine where natural gas is directly injected late in the compression stroke.
Diesel fuel can be used as an ignition source for natural gas. A small amount of diesel fuel injected into the combustion chamber auto-ignites due to the heat of compression producing a flame that then ignites the natural gas. The amount of diesel fuel employed in pilot injections ranges preferably between 5% and 10% of the total fuel consumed on an energy equivalent basis. This solution although effective and employed extensively in the heavy duty trucking industry adds cost and complexity to the engine that now needs to support two fuel systems, which reduces overall fuel system reliability. Two storage vessels for fuel are required along with the associated piping and pumping facilities to deliver these fuels to the fuel injection system on the engine, which now must include either two fuel injectors or a more complicated dual-fuel injector for each cylinder. In markets where operating costs associated with diesel fuel are substantial, such as the heavy duty trucking industry, these added system costs are more than offset by the savings in fuel costs.
Glow plugs are another ignition source for natural gas in direct injection engines. They are finger-shaped pieces of metal that have a heating element in their tip. A surface at the tip heats when a current passes through the heating element due to its electrical resistance and begins to emit light in the visible spectrum, hence the term “glow” plug. After the tip has heated sufficiently, natural gas is injected directly on the surface where it combusts. Normally, the fuel injector has several orifices where natural gas jets emerge during injection events. Since the glow plug is located some distance from the nozzle, only one of these gas jets is ignited by the glow plug on impact. The other gas jets ignite through interaction with the ignited gas jet. This interaction can occur when the other gas jets are diverted towards the ignited jet by hitting the cylinder wall. In this manner it is difficult to control the heat release rate. As a result, the flame produced may not propagate sufficiently to burn all the fuel due to inadequate mixing. Such systems suffer from high unburned hydrocarbon (UHC) emissions and have high cycle-to-cycle variability, which makes it difficult to meet ever more stringent emission regulations. Glow plugs have several failure modes which are generally related to effects from operating temperature and inconsistent combustion. For example, too much electrical power delivered to the glow plug from the engine battery causes excessive heating leading to elevated temperatures beyond what the glow plug is capable of providing. Driving circuit failure can lead to this power overload condition. Poor fuel injection timing, that is fuel injected too early or too late, leads to poor combustion performance which causes carbon deposit build-up on the glow plug surface. General fouling of the glow plug occurs when contaminants in the fuel supply are not properly filtered and are let into the combustion chamber. Both carbon deposit and contaminant build-up phenomena further worsen combustion performance since the heat delivered to the fuel is retarded by the build-up. These failure modes can affect glow plug performance prior to failure which can contribute to even greater emission levels. Glow plugs operate at very high temperatures, for example at 1,350° C., which in general reduces the operating life due to heat fatigue.
The shortcomings of conventional techniques for igniting gaseous fuels in Diesel-cycle engines have limited the market adoption of using these fuels in place of diesel. The present application provides a new and improved apparatus and method for igniting gaseous fuels in Diesel-cycle internal combustion engines.